1. Field of the Invention
The present invention relates to a deflection yoke in a cathode ray tube apparatus, such as a color picture tube, and a cathode ray tube apparatus provided with the same.
2. Description of the Related Art
A color picture tube, for use as a cathode ray tube apparatus, for example, comprises a vacuum envelope that is formed of a glass panel having a substantially rectangular effective portion, a glass funnel coupled to the panel, and a cylindrical glass neck coupled to a small-diameter portion of the funnel. A phosphor screen is formed on the inner surface of the effective portion of the panel. The phosphor screen is composed of dot- or stripe-shaped three-color phosphor layers, which glow blue, green, and red, individually, and a black shielding layer. In the vacuum envelope, a shadow mask that has a large number of electron beam passage apertures is opposed to the phosphor screen. An electron gun that emits three electron beams is located in the neck, and a deflection yoke is mounted on a yoke mounting portion of the funnel. The yoke mounting portion is situated ranging from the outer periphery of the neck to the outer peripheral surface of the funnel.
In the color picture tube constructed in this manner, the three electron beams emitted from the electron gun are deflected in horizontal and vertical directions by horizontal and vertical deflecting magnetic fields that are generated by the deflection yoke, and the electron beams scan the phosphor screen horizontally and vertically through the shadow mask. By doing this, a color image is displayed.
A self-convergence in-line color picture tube is widely practically used as the color picture tube of the aforesaid type. According to this color picture tube, the electron gun is of an in-line type that emits three electron beams that are arranged in a line on the same plane. The deflection yoke is designed to generate a horizontal deflecting magnetic field of the pincushion type and a vertical deflecting magnetic field of the barrel type. The three electron beams that are emitted from the electron gun and arranged in a line can be deflected by the horizontal and vertical deflecting magnetic fields, and the three in-line electron beams can be converged for the entire phosphor screen without requiring use of any special correcting means.
In the color picture tube of this type, on the other hand, the deflection yoke is a substantial source of power consumption. In order to lower the power consumption of a cathode ray tube, therefore, it is essential to reduce the power consumption of the deflection yoke. In recent years, there has been a demand for higher resolution and visibility, and use conditions for high deflecting frequency have been increasing. If the deflection yoke is worked at the high frequency, heat release from the deflection yoke is enormous. The deflecting frequency must be increased in order to match the monitor of an OA apparatus such as an HD (high definition) television or PC (personal computer). Both these circumstances entail increased deflecting power and increased heat release from the deflection yoke.
In order to lower the deflecting power, in general, the neck diameter of the cathode ray tube should be lessened to reduce the outside diameter of the yoke mounting portion on which the deflection yoke is mounted. By doing this, the space on which the deflecting magnetic fields act can be narrowed, so that the deflecting magnetic field can efficiently act on the electron beams.
In the conventional cathode ray tube apparatus that has the yoke mounting portion in the shape of a truncated cone, however, the electron beams are already brought close to the inner surface of the yoke mounting portion of the vacuum envelope when they pass through the envelope. If the neck diameter or the outside diameter of the yoke mounting portion is further reduced, therefore, the electron beams hit the inner surface of the yoke mounting portion before they reach the phosphor screen. Thus, the electron beams inevitably fail to land on some parts of the phosphor screen corresponding to the maximum deflection angle. If the electron beams continue to hit the inner surface of the yoke mounting portion, the hit portions are heated to a temperature high enough to melt glass, so that the vacuum envelope may possibly implode. In the conventional cathode ray tube apparatus, therefore, it is hard to lower the deflecting power by further reducing the neck diameter or the outside diameter of the yoke mounting portion.
If a rectangular raster is generated on the phosphor screen, the region through which the electron beams pass, inside the yoke mounting portion on which the deflection yoke is mounted, also has a substantially rectangular shape. In consideration of this context, the above problems are solved by forming the yoke mounting portion of the funnel so that its shape gradually changes from a circular configuration into a substantially rectangular configuration with distance from the neck or as the panel is approached.
If the yoke mounting portion of the funnel is formed substantially in the shape of a truncated pyramid in this manner, the diameters of the yoke mounting portion in the directions of its major axis (horizontal axis) and minor axis (vertical axis) can be shortened without changing the diameter in the diagonal direction corresponding to the maximum deflection angle. Thus, horizontal and vertical deflecting coils of the deflection yoke can be brought close to the electron beams, so that the electron beams can be efficiently deflected to lower the deflecting power.
There are deflection yokes of various types, including a saddle-saddle-type deflection yoke of which both the horizontal and vertical deflecting coils are of the saddle type, a semi-toroidal deflection yoke having a toroidal vertical deflecting coil, etc. A saddle-saddle-type deflection yoke described in Jpn. Pat. Appln. KOKAI Publication No. 11-265668, for example, comprises a pair of saddle-type horizontal deflecting coils, a pair of saddle-type vertical deflecting coils, and a magnetic core. The horizontal deflecting coils, which are wound in the shape of a truncated pyramid, are located inside an insulating separator. The vertical deflecting coils, which are wound in the shape of a truncated pyramid, are located outside the separator. The core, which has the shape of a truncated pyramid, is provided outside the vertical deflecting coils so as to cover them.
The saddle-saddle-type deflection yoke having the aforesaid basic structure can lower the deflecting power more than the semi-toroidal deflection yoke can. It is hard, however, to manufacture a core of a magnetic material having the shape of a truncated pyramid, and it is also difficult to wind the vertical deflecting coils toroidally around the truncated-pyramid-shaped core. Thus, the deflection yoke entails high cost and lacks in versatility.